The present invention relates to a zirconium alloy having improved corrosion resistance in nitric acid and good creep strength and bending properties. More particularly, it relates to a zirconium alloy which has improved resistance to stress corrosion cracking even in highly concentrated nitric acid solutions having a concentration above the azeotropic concentration at elevated temperatures and which is suitable for use as a structural material in various industrial plants which are exposed to a nitric acid solution, such as plants for the production of nitric acid.
Nitric acid solutions, particularly concentrated nitric acid solutions at an elevated temperature or highly oxidizing nitric acid solutions which contain oxidizing ions such as Cr.sup.6+ ions or Ce.sup.4+ ions cause corrosion of stainless steels to such a great extent that stainless steels are not suitable for use in environments in which they are exposed to these nitric acid solutions. Instead, nonferrous materials are used in these environments.
Titanium, a representative nonferrous corrosion-resistant metallic material, corrodes at a high rate in nitric acid. It is also known that titanium may ignite or explode in fuming nitric acid.
It is well known that zirconium (Zr) exhibits excellent corrosion resistance, particularly resistance to general corrosion and intergranular corrosion in nitric acid environments. Therefore, it has been frequently used as structural materials for industrial plants which are exposed to nitric acid.
Nitric acid and water form an azeotropic mixture at a concentration of 69.8% HNO.sub.3 which corresponds to a specific gravity of 1.42. Thus, aqueous nitric acid solutions have a maximum boiling point of 123.degree. C. at the azeotropic concentration. Due to the formation of the azeotropic mixture, it is not possible to concentrate nitric acid solutions beyond the azeotropic point by ordinary distillation techniques. Therefore, in the commercial production of highly concentrated nitric acid solutions having a concentration above the azeotropic point, a special procedure for concentration such as dehydration with sulfuric acid must be employed.
A plant for the production of highly concentrated nitric acid solutions is inevitably exposed to a concentrated nitric acid having a concentration higher than the azeotropic concentration.
It is known that the behavior of zirconium to corrosion in nitric acid solutions varies significantly when the concentration is increased above the azeotropic concentration. For example, it is reported by Te-Lin Yau in Corrosion, 39 (1983), p. 167 that pure zirconium and its alloys (Zr-1.5Sn and Zr-2.5Nb) are susceptible to stress corrosion cracking (SCC) in a nitric acid solution having a concentration higher than the azeotropic concentration (about 70%). Usually, the corrosion resistance of a metal improves with decreasing temperature, but the above-mentioned SCC susceptibility of zirconium and its alloys can be observed even at room temperature.
Therefore, the corrosion resistance of zirconium and conventional zirconium alloys is not sufficient for them to be used as a structural material for a plant for the production of nitric acid which may be exposed to highly concentrated nitric acid at a concentration above the azeotropic point. Presently, there is no material in the prior art which is known to be suitable for use as a structural material for such a plant. Nitric acid production plants now in use are usually made of either a stainless steel or a nonferrous metal-based alloy, but due to the high corrosion rates of these structural materials, frequent replacement of the equipments and fittings is necessary, which leads to great economic losses.
It is also known that the presence of a large amount of oxidizing ions such as Cr.sup.6+ and Ce.sup.4+ in nitric acid solutions may adversely affect the resistance of zirconium to SCC in nitric acid since the oxidative nature of the solutions is increased. An increase in the oxidative nature of a nitric acid solution also occurs when an additional anodic potential is applied to zirconium to cause anodic polarization, and the resistance of zirconium to SCC in nitric acid may be adversely affected in this case, too.
Japanese Patent Laid-Open Application No. 62-222037(1987) discloses a zirconium alloy containing 0.1-50% Ti. The zirconium alloy has improved resistance to general corrosion in highly oxidizing nitric acid solutions such as those in which a high anodic potential is applied. Such general corrosion is known as corrosion under high potential. However, there is no teaching in the laid-open application about the resistance of the alloy to SCC in nitric acid. Furthermore, it does not suggest the effects of impurities in the alloy nor the criticality of impurity level to attain satisfactory creep strength and SCC resistance.
Japanese Patent Publication No. 33-5704(1958) discloses a corrosion-resistant zirconium alloy containing 1-50% Ti. However, the corrosion resistance referred to in this publication is evaluated in hydrochloric acid and sulfuric acid, and its resistance to corrosion in nitric acid is not disclosed. In addition, there is no reference to the effects of impurities in the alloy, as in the above-mentioned Japanese laid-open application.
It is desirable for structural materials which are used in nitric acid environments to withstand corrosion including general corrosion and stress corrosion cracking not only in highly concentrated nitric acid solutions but in nitric acid solutions of low to medium concentrations at both ambient and elevated temperatures. In addition, the structural materials must have good mechanical properties. However, the corrosion resistance of zirconium metal is insufficient as described above, and its mechanical properties are also unsatisfactory. Specifically, the tensile strength of zirconium is low and its rate of decrease in tensile strength with increasing temperature is higher than that of stainless steels. Moreover, the creep strength of zirconium is not sufficient even in the temperature range of 100.degree.-200.degree. C. which is employed in nitric acid production plants.